1. Field of the Invention
This invention generally relates to the exposure of light on a light-sensitive medium involving a spatial modulator to produce successive columns of individually controlled light spots and particularly relates to electro-optic modulators used in an imaging device for modulating incident light beams, which light is then allowed to reach the light-sensitive medium.
2. Description of the Prior Art
The electro-optic effect, in general, permits extremely rapid and direct modulation of a light phase front with an electronic drive signal.
Various types of electro-optic modulators have been proposed, such as described, for example, in U.S. Pat. Nos. 4,281,904, 4,316,196, 4,804,251, and 4,746,942.
According to U.S. Pat. No. 4,281,904, a TIR (total internal reflection) type of electro-optic device which has each electrode individually addressed is utilized. The operation of a TIR modulator depends on the effect of applying a voltage to a symmetrical electrode pattern to induce a change of the refractive index in an electro-optic element in the region of the surface of the element where the light is totally internally reflected. The electrode pattern is deposited on the surface of the element as an array with the electrodes being arranged parallel to the incident light beam. A voltage is applied to the electrode pattern and induces an electric field adjacent to the surface which alters the refractive index of the element. Thus, incident phase fronts are modulated by the TIR modulator to produce modulated light phase fronts. The electrodes within the electrode pattern are selectively activated in accordance with the desired image pattern.
TIR modulators are also used in U.S. Pat. No. 4,639,073 issued to Yip et al. and U.S. Pat. No. 4,554,561 issued to Daniele et al.
Another electro-optic modulator is the PLZT modulator, which is shown, e.g. in U.S. Pat. No. 4,746,942 to Moulin and U.S. Pat. No. 4,316,196 to Jacobs. The PLZT modulator has a plurality of interleaved electrodes, which, together with a crossed polarizer, forms an array of very small light gates. If a voltage is applied to the electrodes of the PLZT modulator, an electric field is created thus shifting the relative phases of light polarized parallel and perpendicular to the applied field. The plane of polarization of light transmitted to the zones between the electrodes is rotated upon the application of proper voltages to the electrodes.
Hence, electro-optic modulators are used to produce successive columns of individually controlled light spots. Images are produced on the light-sensitive medium by a succession of adjoining bands of spots to produce text and graphics on a film, a printing plate or other medium on which images are to be produced.
In order to avoid any noticeable discontinuity between adjacent bands, it is not only necessary that the relative displacement of the bands and the light-sensitive medium exactly correspond to the size of a column of spots, but also that all the spots be substantially identical in form and intensity. In addition, it is preferred that only the light emerging from the independently selected spot-producing elements of the electro-optic modulator reach the light-sensitive medium at the imaging plane.
To achieve desired uniformity between selected spots, all the selectable elements or gates of the modulator must be uniformly illuminated. This can better be achieved by illuminating an area larger than the zone occupied by the selectable modulator elements in order to compensate for the decrease in intensity of the incident light at the edges of the light phase front. It is then desirable to prevent the extraneous radiation overlapping said zone because of misalignment or for other reasons from reaching the light-sensitive medium.
In general, multi-electrodes modulating systems associated with a light sensitive medium for imaging do not allow light (or other radiation) to reach the medium in the absence of energizing selected electrodes. In these systems, the light intensity of the spots reaching the medium is obtained by rays that have incurred a loss of energy caused by the modulating system as they pass through the modulator material. They can generally produce good image contrast, but at the expense of efficiency. Such systems may include deformable mirrors, crossed polarizers, deflection by diffraction. For imaging supports requiring higher radiant energy such as heat-sensitive polymer printing plates, it is desirable to lose as little energy as possible through the modulator, even at the expense of a loss of contrast. This can be achieved by letting light beams, unimpeded by the modulator, reach the sensitive medium. In this approach, the modulator electrodes are normally inactivated to allow all the energy from the incident light beams to reach the medium. Any activated electrode will block the beam it controls. In other words, all the electrodes are activated when no light should reach the medium. In this alternative, the illuminated area reaching the modulator should be exactly confined to the imaging electrode area of the modulator to avoid the influence of leakage of marginal rays that would expose the light-sensitive medium. The uniformity in illumination or exposure of the light-sensitive medium would be negatively affected by these marginal rays reaching the active zone of the medium at its edges.
The insertion of a mask to limit the illumination to the active zone, although simple in appearance, presents difficulties of implementation and the marginal rays adjacent to the ends of the imaging zone are affected by diffraction by the mask edges.